The human macrophage, an important component of the immune system, in large part functions to detect and damage foreign cells. The interaction between human macrophages and human erythrocyte target cells serves as a study model for immune macrophage-lymphoblastoid cell interaction. Human erythrocytes coated with IgG, C3 or concanavalin-A have been observed to bind to homologous macrophages and become damaged over time. The role of antigen in this recognition as well as the effect of pharmcologic agents, such as solubilized corticosteroid analogues, levamisole and BCG are being studied. Altering macrophage membrane fluidity and target cell membrane fluidity by altering the cholesterol-phospholipid ratio of the cell is also being studied as well as the effect of altered structural and biochemical targets in macrophage recognition. Macrophage-lymphoblastoid cell interaction is being examined as the model for macrophage-tumor cell interaction. A spontaneous interaction has been observed and is being characterized. The nature of the locus on the lymphoblastoid cell surface recognized by the macrophage has been shown to be independent of IgG and C3b, but its precise nature is as yet undefined. Delineation of the mechanism involved in macrophage recognition and recognition of altered cell surfaces should provide insight leading to new approaches for harnessing the macrophages ability to detect tumor cells and modify their viability.